On the simplest level, science is knowledge of the world of nature. There are many regularities in nature that humankind has had to recognize for survival since the emergence of Homo sapiens as a species. The Sun and the Moon periodically repeat their movements. Some motions, like the daily “motion” of the Sun, are simple to observe, while others, like the annual “motion” of the Sun, are far more difficult. Both motions correlate with important terrestrial events. Day and night provide the basic rhythm of human existence. The seasons determine the migration of animals upon which humans have depended for millennia for survival. With the invention of agriculture, the seasons became even more crucial, for failure to recognize the proper time for planting could lead to starvation. Science defined simply as knowledge of natural processes is universal among humankind, and it has existed since the dawn of human existence.

The mere recognition of regularities does not exhaust the full meaning of science, however. In the first place, regularities may be simply constructs of the human mind. Humans leap to conclusions. The mind cannot tolerate chaos, so it constructs regularities even when none objectively exists. Thus, for example, one of the astronomical “laws” of the Middle Ages was that the appearance of comets presaged a great upheaval, as the Norman Conquest of Britain followed the comet of 1066. True regularities must be established by detached examination of data. Science, therefore, must employ a certain degree of skepticism to prevent premature generalization.

Regularities, even when expressed mathematically as laws of nature, are not fully satisfactory to everyone. Some insist that genuine understanding demands explanations of the causes of the laws, but it is in the realm of causation that there is the greatest disagreement. Modern quantum mechanics, for example, has given up the quest for causation and today rests only on mathematical description. Modern biology, on the other hand, thrives on causal chains that permit the understanding of physiological and evolutionary processes in terms of the physical activities of entities such as molecules, cells, and organisms. But even if causation and explanation are admitted as necessary, there is little agreement on the kinds of causes that are permissible, or possible, in science. If the history of science is to make any sense whatsoever, it is necessary to deal with the past on its own terms, and the fact is that for most of the history of science natural philosophers appealed to causes that would be summarily rejected by modern scientists. Spiritual and divine forces were accepted as both real and necessary until the end of the 18th century and, in areas such as biology, deep into the 19th century as well.

Certain conventions governed the appeal to God or the gods or to spirits. Gods and spirits, it was held, could not be completely arbitrary in their actions. Otherwise, the proper response would be propitiation, not rational investigation. But, since the deity or deities were themselves rational or bound by rational principles, it was possible for humans to uncover the rational order of the world. Faith in the ultimate rationality of the creator or governor of the world could actually stimulate original scientific work. Kepler’s laws, Newton’s absolute space, and Einstein’s rejection of the probabilistic nature of quantum mechanics were all based on theological, not scientific, assumptions. For sensitive interpreters of phenomena, the ultimate intelligibility of nature has seemed to demand some rational guiding spirit. A notable expression of this idea is Einstein’s statemet that the wonder is not that humankind comprehends the world but that the world is comprehensible.

Science, then, is to be considered in this article as knowledge of natural regularities that is subjected to some degree of skeptical rigour and explained by rational causes. One final caution is necessary. Nature is known only through the senses, of which sight, touch, and hearing are the dominant ones, and the human notion of reality is skewed toward the objects of these senses. The invention of such instruments as the telescope, the microscope, and the Geiger counter enabled an ever-increasing range of phenomena within the scope of the senses. Thus, scientific knowledge of the world is only partial, and the progress of science follows the ability of humans to make phenomena perceivable.

This article provides a broad survey of the development of science as a way of studying and understanding the world, from the primitive stage of noting important regularities in nature to the epochal revolution in the notion of what constitutes reality that occurred in 20th-century physics. More-detailed treatments of the histories of specific sciences, including developments of the later 20th and early 21st centuries, may be found in the articles biology; Earth science; and physical science.

Science as natural philosophy

Precritical science

Science, as it has been defined above, made its appearance before writing. It is necessary, therefore, to infer from archaeological remains what was the content of that science. From cave paintings and from apparently regular scratches on bone and reindeer horn, it is known that prehistoric humans were close observers of nature who carefully tracked the seasons and times of the year. About 2500 bce there was a sudden burst of activity that seems to have had clear scientific importance. Great Britain and northwestern Europe contain large stone structures from that era, the most famous of which is Stonehenge on the Salisbury Plain in England, that are remarkable from a scientific point of view. Not only do they reveal technical and social skills of a high order—it was no mean feat to move such enormous blocks of stone considerable distances and place them in position—but the basic conception of Stonehenge and the other megalithic structures also seems to combine religious and astronomical purposes. Their layouts suggest a degree of mathematical sophistication that was first suspected only in the mid-20th century. Stonehenge is a circle, but some of the other megalithic structures are egg-shaped and, apparently, constructed on mathematical principles that require at least practical knowledge of the Pythagorean theorem that the square of the hypotenuse of a right triangle is equal to the sum of the squares of the other two sides. This theorem, or at least the Pythagorean numbers that can be generated by it, seems to have been known throughout Asia, the Middle East, and Neolithic Europe two millennia before the birth of Pythagoras.

This combination of religion and astronomy was fundamental to the early history of science. It is found in Mesopotamia, Egypt, China (although to a much lesser extent than elsewhere), Central America, and India. The spectacle of the heavens, with the clearly discernible order and regularity of most heavenly bodies highlighted by extraordinary events such as comets and novae and the peculiar motions of the planets, obviously was an irresistible intellectual puzzle to early humankind. In its search for order and regularity, the human mind could do no better than to seize upon the heavens as the paradigm of certain knowledge. Astronomy was to remain the queen of the sciences (welded solidly to theology) for the next 4,000 years.

Science, in its mature form, developed only in the West. But it is instructive to survey the protoscience that appeared in other areas, especially in light of the fact that until quite recently this knowledge was often, as in China, far superior to Western science.

China

As has already been noted, astronomy seems everywhere to have been the first science to emerge. Its intimate relation to religion gave it a ritual dimension that then stimulated the growth of mathematics. Chinese savants, for example, early devised a calendar and methods of plotting the positions of stellar constellations. Since changes in the heavens presaged important changes on the Earth (for the Chinese considered the universe to be a vast organism in which all elements were connected), astronomy and astrology were incorporated into the system of government from the very dawn of the Chinese state in the 2nd millennium bce. As the Chinese bureaucracy developed, an accurate calendar became absolutely necessary to the maintenance of legitimacy and order. The result was a system of astronomical observations and records unparalleled elsewhere, thanks to which there are, today, star catalogs and observations of eclipses and novae that go back for millennia.

In other sciences too the overriding emphasis was on practicality, for the Chinese, almost alone among ancient peoples, did not fill the cosmos with gods and demons whose arbitrary wills determined events. Order was inherent and, therefore, expected. It was for humans to detect and describe this order and to profit from it. Chemistry (or, rather, alchemy), medicine, geology, geography, and technology were all encouraged by the state and flourished. Practical knowledge of a high order permitted the Chinese to deal with practical problems for centuries on a level not attained in the West until the Renaissance.

India

Astronomy was studied in India for calendrical purposes to set the times for both practical and religious tasks. Primary emphasis was placed on solar and lunar motions, the fixed stars serving as a background against which these luminaries moved. Indian mathematics seems to have been quite advanced, with particular sophistication in geometrical and algebraic techniques. This latter branch was undoubtedly stimulated by the flexibility of the Indian system of numeration that later was to come into the West as the Hindu-Arabic numerals.

America

Quite independently of China, India, and the other civilizations of Europe and Asia, the Maya of Central America, building upon older cultures, created a complex society in which astronomy and astrology played important roles. Determination of the calendar, again, had both practical and religious significance. Solar and lunar eclipses were important, as was the position of the bright planet Venus. No sophisticated mathematics are known to have been associated with this astronomy, but the Mayan calendar was both ingenious and the result of careful observation.

The Middle East

In the cradles of Western civilization in Egypt and Mesopotamia, there were two rather different situations. In Egypt there was an assumption of cosmic order guaranteed by a host of benevolent gods. Unlike China, whose rugged geography often produced disastrous floods, earthquakes, and violent storms that destroyed crops, Egypt was surpassingly placid and delightful. Egyptians found it difficult to believe that all ended with death. Enormous intellectual and physical labour, therefore, was devoted to preserving life after death. Both Egyptian theology and the pyramids are testaments to this preoccupation. All of the important questions were answered by religion, so the Egyptians did not concern themselves overmuch with speculations about the universe. The stars and the planets had astrological significance in that the major heavenly bodies were assumed to “rule” the land when they were in the ascendant (from the succession of these “rules” came the seven-day week, after the five planets and the Sun and the Moon), but astronomy was largely limited to the calendrical calculations necessary to predict the annual life-giving flood of the Nile. None of this required much mathematics, and there was, consequently, little of any importance.

Mesopotamia was more like China. The life of the land depended upon the two great rivers, the Tigris and the Euphrates, as that of China depended upon the Huang He (Yellow River) and the Yangtze (Chang Jiang). The land was harsh and made habitable only by extensive damming and irrigation works. Storms, insects, floods, and invaders made life insecure. To create a stable society required both great technological skill, for the creation of hydraulic works, and the ability to hold off the forces of disruption. These latter were early identified with powerful and arbitrary gods who dominated Mesopotamian theology. The cities of the plain were centred on temples run by a priestly caste whose functions included the planning of major public works, like canals, dams, and irrigation systems, the allocation of the resources of the city to its members, and the averting of a divine wrath that could wipe everything out.

Mathematics and astronomy thrived under these conditions. The number system, probably drawn from the system of weights and coinage, was based on 60 (it was in ancient Mesopotamia that the system of degrees, minutes, and seconds developed) and was adapted to a practical arithmetic. The heavens were the abode of the gods, and because heavenly phenomena were thought to presage terrestrial disasters, they were carefully observed and recorded. Out of these practices grew, first, a highly developed mathematics that went far beyond the requirements of daily business, and then, some centuries later, a descriptive astronomy that was the most sophisticated of the ancient world until the Greeks took it over and perfected it.

Nothing is known of the motives of these early mathematicians for carrying their studies beyond the calculations of volumes of dirt to be removed from canals and the provisions necessary for work parties. It may have been simply intellectual play—the role of playfulness in the history of science should not be underestimated—that led them onward to abstract algebra. There are texts from about 1700 bce that are remarkable for their mathematical suppleness. Babylonian mathematicians knew the Pythagorean relationship well and used it constantly. They could solve simple quadratic equations and could even solve problems in compound interest involving exponents. From about a millennium later there are texts that utilize these skills to provide a very elaborate mathematical description of astronomical phenomena.

Although China and Mesopotamia provide examples of exact observation and precise description of nature, what is missing is explanation in the scientific mode. The Chinese assumed a cosmic order that was vaguely founded on the balance of opposite forces (yin–yang) and the harmony of the five elements (water, wood, metal, fire, and earth). Why this harmony obtained was not discussed. Similarly, the Egyptians found the world harmonious because the gods willed it so. For Babylonians and other Mesopotamian cultures, order existed only so long as all-powerful and capricious gods supported it. In all these societies, humans could describe nature and use it, but to understand it was the function of religion and magic, not reason. It was the Greeks who first sought to go beyond description and to arrive at reasonable explanations of natural phenomena that did not involve the arbitrary will of the gods. Gods might still play a role, as indeed they did for centuries to come, but even the gods were subject to rational laws.

Greek science

The birth of natural philosophy

There seems to be no good reason why the Hellenes, clustered in isolated city-states in a relatively poor and backward land, should have struck out into intellectual regions that were only dimly perceived, if at all, by the splendid civilizations of the Yangtze, Tigris and Euphrates, and Nile valleys. There were many differences between ancient Greece and the other civilizations, but perhaps the most significant was religion. What is striking about Greek religion, in contrast to the religions of Mesopotamia and Egypt, is its puerility. Both of the great river civilizations evolved complex theologies that served to answer most, if not all, of the large questions about humankind’s place and destiny. Greek religion did not. It was, in fact, little more than a collection of folk tales, more appropriate to the campfire than to the temple. Perhaps this was the result of the collapse of an earlier Greek civilization, the Mycenaean, toward the end of the 2nd millennium bce, when the Dark Age descended upon Greece and lasted for three centuries. All that was preserved were stories of gods and men, passed along by poets, that dimly reflected Mycenaean values and events. Such were the great poems of Homer, the Iliad and the Odyssey, in which heroes and gods mingled freely with one another. Indeed, they mingled too freely, for the gods appear in these tales as little more than immortal adolescents whose tricks and feats, when compared with the concerns of a Marduk or Jehovah, are infantile. There really was no Greek theology in the sense that theology provides a coherent and profound explanation of the workings of both the cosmos and the human heart. Hence, there were no easy answers to inquiring Greek minds. The result was that ample room was left for a more penetrating and ultimately more satisfying mode of inquiry. Thus were philosophy and its oldest offspring, science, born.

The first natural philosopher, according to Hellenic tradition, was Thales of Miletus, who flourished in the 6th century bce. We know of him only through later accounts, for nothing he wrote has survived. He is supposed to have predicted a solar eclipse in 585 bce and to have invented the formal study of geometry in his demonstration of the bisecting of a circle by its diameter. Most importantly, he tried to explain all observed natural phenomena in terms of the changes of a single substance, water, which can be seen to exist in solid, liquid, and gaseous states. What for Thales guaranteed the regularity and rationality of the world was the innate divinity in all things that directed them to their divinely appointed ends. From these ideas there emerged two characteristics of classical Greek science. The first was the view of the universe as an ordered structure (the Greek kósmos means “order”). The second was the conviction that this order was not that of a mechanical contrivance but that of an organism: all parts of the universe had purposes in the overall scheme of things, and objects moved naturally toward the ends they were fated to serve. This motion toward ends is called teleology and, with but few exceptions, it permeated Greek as well as much later science.

Thales inadvertently made one other fundamental contribution to the development of natural science. By naming a specific substance as the basic element of all matter, Thales opened himself to criticism, which was not long in coming. His own disciple, Anaximander, was quick to argue that water could not be the basic substance. His argument was simple: water, if it is anything, is essentially wet; nothing can be its own contradiction. Hence, if Thales were correct, the opposite of wet could not exist in a substance, and that would preclude all of the dry things that are observed in the world. Therefore, Thales was wrong. Here was the birth of the critical tradition that is fundamental to the advance of science.

Thales’ conjectures set off an intellectual explosion, most of which was devoted to increasingly refined criticisms of his doctrine of fundamental matter. Various single substances were proposed and then rejected, ultimately in favour of a multiplicity of elements that could account for such opposite qualities as wet and dry, hot and cold. Two centuries after Thales, most natural philosophers accepted a doctrine of four elements: earth (cold and dry), fire (hot and dry), water (cold and wet), and air (hot and wet). All bodies were made from these four.

The presence of the elements only guaranteed the presence of their qualities in various proportions. What was not accounted for was the form these elements took, which served to differentiate natural objects from one another. The problem of form was first attacked systematically by the philosopher and cult leader Pythagoras in the 6th century bce. Legend has it that Pythagoras became convinced of the primacy of number when he realized that the musical notes produced by a monochord were in simple ratio to the length of the string. Qualities (tones) were reduced to quantities (numbers in integral ratios). Thus was born mathematical physics, for this discovery provided the essential bridge between the world of physical experience and that of numerical relationships. Number provided the answer to the question of the origin of forms and qualities.

Aristotle and Archimedes

Hellenic science was built upon the foundations laid by Thales and Pythagoras. It reached its zenith in the works of Aristotle and Archimedes. Aristotle represents the first tradition, that of qualitative forms and teleology. He was himself a biologist whose observations of marine organisms were unsurpassed until the 19th century. Biology is essentially teleological—the parts of a living organism are understood in terms of what they do in and for the organism—and Aristotle’s biological works provided the framework for the science until the time of Charles Darwin. In physics, teleology is not so obvious, and Aristotle had to impose it on the cosmos. From Plato, his teacher, he inherited the theological proposition that the heavenly bodies (stars and planets) are literally divine and, as such, perfect. They could, therefore, move only in perfect, eternal, unchanging motion, which, by Plato’s definition, meant perfect circles. The Earth, being obviously not divine, and inert, was at the centre. From the Earth to the sphere of the Moon, all things constantly changed, generating new forms and then decaying back into formlessness. Above the Moon the cosmos consisted of contiguous and concentric crystalline spheres moving on axes set at angles to one another (this accounted for the peculiar motions of the planets) and deriving their motion either from a fifth element that moved naturally in circles or from heavenly souls resident in the celestial bodies. The ultimate cause of all motion was a prime, or unmoved, mover (God) that stood outside the cosmos.

Aristotle was able to make a great deal of sense of observed nature by asking of any object or process: what is the material involved, what is its form and how did it get that form, and, most important of all, what is its purpose? What should be noted is that, for Aristotle, all activity that occurred spontaneously was natural. Hence, the proper means of investigation was observation. Experiment, that is, altering natural conditions in order to throw light on the hidden properties and activities of objects, was unnatural and could not, therefore, be expected to reveal the essence of things. Experiment was thus not essential to Greek science.

The problem of purpose did not arise in the areas in which Archimedes made his most important contributions. He was, first of all, a brilliant mathematician whose work on conic sections and on the area of the circle prepared the way for the later invention of the calculus. It was in mathematical physics, however, that he made his greatest contributions to science. His mathematical demonstration of the law of the lever was as exact as a Euclidean proof in geometry. Similarly, his work on hydrostatics introduced and developed the method whereby physical characteristics, in this case specific gravity, which Archimedes discovered, are given mathematical shape and then manipulated by mathematical methods to yield mathematical conclusions that can be translated back into physical terms.

In one major area the Aristotelian and the Archimedean approaches were forced into a rather inconvenient marriage. Astronomy was the dominant physical science throughout antiquity, but it had never been successfully reduced to a coherent system. The Platonic-Aristotelian astral religion required that planetary orbits be circles. But, particularly after the conquests of Alexander the Great had made the observations and mathematical methods of the Babylonians available to the Greeks, astronomers found it impossible to reconcile theory and observation. Astronomy then split into two parts: one was physical and accepted Aristotelian theory in accounting for heavenly motion, and the other ignored causation and concentrated solely on the creation of a mathematical model that could be used for computing planetary positions. Ptolemy, in the 2nd century ce, carried the latter tradition to its highest point in antiquity in his Hē mathēmatikē syntaxis (“The Mathematical Collection,” better known under its Greek-Arabic title, Almagest).

Medicine

The Greeks not only made substantial progress in understanding the cosmos but also went far beyond their predecessors in their knowledge of the human body. Pre-Greek medicine had been almost entirely confined to religion and ritual. Disease was considered the result of divine disfavour and human sin, to be dealt with by spells, prayers, and other propitiatory measures. In the 5th century bce a revolutionary change came about that is associated with the name of Hippocrates. It was Hippocrates and his school who, influenced by the rise of natural philosophy, first insisted that disease was a natural, not a supernatural, phenomenon. Even maladies as striking as epilepsy, whose seizures appeared to be divinely caused, were held to originate in natural causes within the body.

The height of medical science in antiquity was reached late in the Hellenistic period. Much work was done at the museum of Alexandria, a research institute set up under Greek influence in Egypt in the 3rd century bce to sponsor learning in general. The heart and the vascular system were investigated, as were the nerves and the brain. The organs of the thoracic cavity were described, and attempts were made to discover their functions. It was on these researches, and on his own dissections of apes and pigs, that the last great physician of antiquity, Galen of Pergamum, based his physiology. It was, essentially, a tripartite system in which so-called spirits—natural, vital, and animal—passed respectively through the veins, the arteries, and the nerves to vitalize the body as a whole. Galen’s attempts to correlate therapeutics with his physiology were not successful, and so medical practice remained eclectic and a matter of the physician’s choice. Usually the optimal choice was that propounded by the Hippocratics, who relied primarily on simple, clean living and the ability of the body to heal itself.

Science in Rome and Christianity

The apogee of Greek science in the works of Archimedes and Euclid coincided with the rise of Roman power in the Mediterranean. The Romans were deeply impressed by Greek art, literature, philosophy, and science, and after their conquest of Greece many Greek intellectuals served as household slaves tutoring noble Roman children. The Romans were a practical people, however, and, while they contemplated the Greek intellectual achievement with awe, they also could not help but ask what good it had done the Greeks. Roman common sense was what kept Rome great; science and philosophy were either ignored or relegated to rather low status. Even such a Hellenophile as the statesman and orator Cicero used Greek thought more to buttress the old Roman ways than as a source of new ideas and viewpoints.

The spirit of independent research was quite foreign to the Roman mind, so scientific innovation ground to a halt. The scientific legacy of Greece was condensed and corrupted into Roman encyclopaedias whose major function was entertainment rather than enlightenment. Typical of this spirit was the 1st-century-ce aristocrat Pliny the Elder, whose Natural History was a multivolume collection of myths, odd tales of wondrous creatures, magic, and some science, all mixed together uncritically for the titillation of other aristocrats. Aristotle would have been embarrassed by it.

At its height Rome incorporated a host of peoples with different customs, languages, and religions within its empire. One religious sect that proved more significant than the rest was Christianity. Jesus and his kingdom were not of this world, but his disciples and their followers were. This world could not be ignored, even though concern with worldly things could be dangerous to the soul. So the early Christians approached the worldly wisdom of their time with ambivalence: on the one hand, the rhetoric and the arguments of ancient philosophy were snares and delusions that might mislead the simple and the unwary; on the other hand, the sophisticated and the educated of the empire could not be converted unless the Christian message was presented in the terms and rhetoric of the philosophical schools. Before they knew it, the early Christians were enmeshed in metaphysical arguments, some of which involved physics. What, for example, was the nature of Jesus, in purely physical terms? How was it possible that anybody could have two different essential natures, as was claimed for Jesus? Such questions revealed how important knowledge of the arguments of Greek thinkers on the nature of substance could be to those engaged in founding a new theology.

Ancient learning, then, did not die with the fall of Rome and the occupation of the Western Empire by tribes of Germanic barbarians. To be sure, the lamp of learning burned very feebly, but it did not go out. Monks in monasteries faithfully copied out classics of ancient thought and early Christianity and preserved them for posterity. Monasteries continued to teach the elements of ancient learning, for little beyond the elementary survived in the Latin West. In the East the Byzantine Empire remained strong, and there the ancient traditions continued. There was little original work done in the millennium following the fall of Rome, but the ancient texts were preserved along with knowledge of the ancient Greek language. This was to be a precious reservoir of learning for the Latin West in later centuries.

Science in Islam

The torch of ancient learning passed first to one of the invading groups that helped bring down the Eastern Empire. In the 7th century the Arabs, inspired by their new religion, burst out of the Arabian peninsula and laid the foundations of an Islamic empire that eventually rivalled that of ancient Rome. To the Arabs, ancient science was a precious treasure. The Qurʿān, the sacred book of Islam, particularly praised medicine as an art close to God. Astronomy and astrology were believed to be one way of glimpsing what God willed for humankind. Contact with Hindu mathematics and the requirements of astronomy stimulated the study of numbers and of geometry. The writings of the Hellenes were, therefore, eagerly sought and translated, and thus much of the science of antiquity passed into Islamic culture. Greek medicine, Greek astronomy and astrology, and Greek mathematics, together with the great philosophical works of Plato and, particularly, Aristotle, were assimilated in Islam by the end of the 9th century. Nor did the Arabs stop with assimilation. They criticized and they innovated. Islamic astronomy and astrology were aided by the construction of great astronomical observatories that provided accurate observations against which the Ptolemaic predictions could be checked. Numbers fascinated Islamic thinkers, and this fascination served as the motivation for the creation of algebra (from Arabic al-jabr) and the study of algebraic functions.

Medieval European science

Medieval Christendom confronted Islam chiefly in military crusades, in Spain and the Holy Land, and in theology. From this confrontation came the restoration of ancient learning to the West. The Reconquista in Spain gradually pushed the Moors south from the Pyrenees, and among the treasures left behind were Arabic translations of Greek works of science and philosophy. In 1085 the city of Toledo, with one of the finest libraries in Islam, fell to the Christians. Among the occupiers were Christian monks who quickly began the process of translating ancient works into Latin. By the end of the 12th century much of the ancient heritage was again available to the Latin West.

The medieval world was caricatured by thinkers of the 18th-century Enlightenment as a period of darkness, superstition, and hostility to science and learning. On the contrary, it was one of great technological vitality. The advances that were made may appear today as trifling, but that is because they were so fundamental. They included the horseshoe and the horse collar, without which horsepower cannot be efficiently exploited. The invention of the crank, the brace and bit, the wheelbarrow, and the flying buttress made possible the great Gothic cathedrals. Improvements in the gear trains of waterwheels and the development of windmills harnessed these sources of power with great efficiency. Mechanical ingenuity, building on experience with mills and power wheels, culminated in the 14th century in the mechanical clock, which not only set a new standard of chronometrical accuracy but also provided philosophers with a new metaphor for nature itself.

An equal amount of energy was devoted to achieving a scientific understanding of nature, but it is essential to understand to what use medieval thinkers put this kind of knowledge. As the fertility of the technology shows, medieval Europeans had no deep prejudices against utilitarian knowledge. But the areas in which scientific knowledge could find useful expression were few. Instead, science was viewed chiefly as a means of understanding God’s creation and, thereby, the Godhead itself. The best example of this attitude is found in the medieval study of optics. Light, as Genesis makes clear, was among the first creations of God. The 12th–13th-century cleric-scholar Robert Grosseteste saw in light the first creative impulse. As light spread, it created both space and matter, and, in its reflection from the outermost circle of the cosmos, it gradually solidified into the heavenly spheres. To understand the laws of the propagation of light was to understand, in some slight way, the nature of the creation.

In the course of studying light, particular problems were isolated and attacked. What, for example, is the rainbow? It is impossible to get close enough to a rainbow to see clearly what is going on, for as the observer moves, so too does the rainbow. It does seem to depend upon the presence of raindrops, so medieval investigators sought to bring the rainbow down from the skies into their studies. Insight into the nature of the rainbow could be achieved by simulating the conditions under which rainbows occur. For raindrops the investigators substituted hollow glass balls filled with water, so that the rainbow could be studied at leisure. Valid conclusions about rainbows could then be drawn by assuming the validity of the analogy between raindrops and water-filled globes. This involved the implicit assumptions that nature was simple (i.e., governed by a few general laws) and that similar effects had similar causes. Such a nature was what could be expected of a rational, benevolent deity. Hence, the assumption could be persuasively adopted.

Medieval philosophers were not content, as the above example shows, to repeat what the ancients had said. They subjected ancient texts to close critical scrutiny. Usually, the intensity of the criticism was directly proportional to the theological significance of the problem involved. Such was the case with motion. Medieval philosophers examined all aspects of motion with great care, for the nature of motion had important theological implications. Thomas Aquinas used Aristotle’s dictum, that everything that moves is moved by something else, to show that God must exist, for otherwise the existence of any motion would imply an infinite regression of prior causal motions.

It should be clear that there was no conscious conflict between science and religion in the Middle Ages. As Aquinas pointed out, God was the author of both the book of Scripture and the book of nature. The guide to nature was reason, the faculty that was the image of God in which humankind was made. Scripture was direct revelation, although it needed interpretation, for there were passages that were obscure or difficult. The two books, having the same author, could not contradict each other. For the short term, science and revelation marched hand in hand. Aquinas carefully wove knowledge of nature into his theology, as in his proof from motion of the existence of God. But if his scientific concepts of motion should ever be challenged, there would necessarily be a theological challenge as well. By working science into the very fabric of his theology, he virtually guaranteed that someday there would be conflict. Theologians would side with theology and scientists with science, to create a breach that neither particularly desired.

The glory of medieval science was its integration of science, philosophy, and theology into a magnificent and comprehensible whole. It can be best contemplated in the greatest of all medieval poems, Dante’s Divine Comedy. Here was an essentially Aristotelian cosmos, finite and easily understood, over which God, his Son, and his saints reigned. Humanity and Earth occupied the centre, as befitted their centrality in God’s plan. The nine circles of hell were populated by humans whose exercise of their free will had led to their damnation. Purgatory contained lesser sinners still capable of salvation. The heavenly spheres were populated by the saved and the saintly. The natural hierarchy gave way to the spiritual hierarchy as one ascended toward the throne of God. Such a hierarchy was reflected in the social and political institutions of medieval Europe, and God, the supreme monarch, ruled his creation with justice and love. All fit together in a grand cosmic scheme, one not to be abandoned lightly.

Britannica Web sites

The Latin word scientia, which means "knowing" or "being skilled," is the source of the English word science. It has become common, especially in school curricula, to restrict the usage of the word science to the study of the physical, earth, space, and life sciences-for example, physics, chemistry, geology, astronomy, biology, and anatomy.

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